The invention relates to current detectors, and more particularly, a current detector which can detect the existence of an active antenna from the currents in a satellite communication system.
FIG. 1 shows a diagram of satellite receiver including an active antenna. Generally, the satellite receiver 12 and active antenna 14 are bought or installed separately. Whenever an active antenna 14 is mounted or not on the satellite receiver 12, the satellite receiver 12 must detect the existence of antenna and inform the user. For example, an antenna detector may act as shown in Table 1.
TABLE 1The action of an antenna detectorAntenna Statusnot mountedmountedOutput logic levelLogic HighLogic Lowof antenna detector
An active antenna is with a power supply for receiving satellite signals, while a passive antenna receives satellite signals without any power supply.
FIG. 2 shows a conventional antenna detector 20. The antenna detector 20 comprises an operation amplifier 22 and a resistor 24. The amplifier 22 is coupled to a DC power Vcc and ground. The non-inverting input of the operational amplifier 22 is coupled to one end of the resistor 24, and the inverting input of the operational amplifier 22 is coupled to the other end of the resistor 24. The inverting input is also coupled to a load circuit 26. A load current ILOAD through load circuit 26 represents the current for driving an active antenna. The load current ILOAD through the resistor 24 causes the voltage levels of the two inputs of the operational amplifier 22 to be different. The operational amplifier 22 amplifies the voltage difference, and the output of the operational amplifier 22 is at logic high. When no current is passing through the load circuit 26, the active antenna has not been plugged into the satellite receiver. Thus, the voltage difference across two inputs of the operational amplifier 22 is zero, which means the output of the operational amplifier 22 is at logic low.
Conventional antenna detector 20 seems to meet the requirements set in Table 1. The voltage of the power supply of a typical active antenna, however, exceeds 6 volts. When Vcc is 6 volts, and an active antenna is plugged into the antenna detector, the output voltage level of the operational amplifier is nearly 6 volts. The operating range of typical logic circuit, however, is 0-3.3 only volts. This means, 3.3 volts are “logic high” as defined by the logic circuit, and a signal with potential of 6 volts may destroy the logic circuit. Thus, the conventional antenna detector fails to meet the requirements listed in Table 1.